docs: def fulfill(self, agreement_id, reward_address, amount, account):
          """
          Fulfill the lock reward condition.
  
          :param agreement_id: id of the agreement, hex str
          :param reward_address: ethereum account address, hex str
          :param amount: Amount of tokens, int
          :param account: Account instance
          :return:
          """
          return self._fulfill(
              agreement_id,
              reward_address,
              amount,
              transact={'from': account.address,
                        'passphrase': account.password}
          )